Adults possess a rich set of conceptions about the internal cognitions and mental states of themselves and others. These conceptions constitute a naive theory of mind and as such are crucial to a mature understanding of self and others. But, what, if anything, do young children know about the mind? The proposed research investigates the nature of children's conception of mental phenomena in three ways. First it examines how children distinguish mental entities, such as dreams and thoughts from obviously real entities -- such as rocks, trees, and tables -- and from less obviously real entities -- such as shadows, pictures, and pains. Second, it examines when children's conception of human action reflects a theory of mind as revealed in an understanding of beliefs and desires as causes of action. Third, the research investigates how children acquire such knowledge and conceptions. The process is one of guided discovery within a social environment rich in information and indirect instruction, a process termed the socialization of cognition. A variety of methods -- experimental, quasi-naturalistic, and naturalistic -- will be used in order to (a) overcome methodological obstacles to research with young children and (b) to provide a rich set of converging findings on the topics of interest. In all, four series of experiments are proposed. These encompass (1) experimental studies of young children's understanding of the differences between real and mental entities, (2) natural language analyses of such children's talk about mental phenomena, with special emphasis on children's understanding of desires and beliefs, (3) investigations of how children's concepts and theories on these matters are socialized, based on examining the information available to them in the speech of adults, and (4) two preliminary studies of very young children's (16-36 mos.) development of such concepts and terms.